1. Field of Invention
The present invention relates, in general, to a method for cleaning metal masks used in fabricating integrated circuits. More particularly, the present invention relates to a method for electrochemical cleaning, of the metal residue which forms on molybdenum masks during processing of integrated circuits, using a phosphoric acid-based solution.
2. Description of Related Art
Often, metal masks are used repeatedly and cyclically in integrated circuit processing. Consequent metallic residue (build-up or stack) is one source of problematic mask defects. After a given cycle of processing is completed, therefore, the mask is separated from the wafer substrate and chemically cleaned to remove the metallurgical stack. Such cleaning leaves behind a layer of metallic residue which stains the mask. Those stains influence the via size of the mask; thus, they limit the number of times (cycles) the mask can be used. Moreover, the polyimide layer of the substrate (which acts as an intermetal dielectric) has been found to contain particles of metal embedded in its surface after processing with stained masks.
In fabricating controlled collapsible chip connection ("C4") technology, molybdenum masks are generally used. Such masks present an additional problem for the typical chemical cleaning process given masks: the process must remove the metal residue without attacking the base molybdenum. It has been found that conventional chemical cleaning processes are unable to address that problem satisfactorily.
With the above discussion in mind, it is one object of the present invention to provide an improved process for removing metal residue from molybdenum masks without chemically attacking the molybdenum. A second object is to assure that the process is adaptable to manufacturing needs. A related object is to provide a process which is fast, on the order of one or two minutes. Also of advantage, and a further object, is a process which increases the number of cycles for which a given mask can be used without requiring further cleaning.